Prior to the present invention, manufacturers of refrigeration systems, such as the refrigerated display and storage systems for supermarkets, have been installing separate current sensing relays to perform the functions of protecting against nuisance tripping of the oil pressure safety switch and sensing current in the defrost circuit to determine when the defrost cycle should be terminated. When a control panel required both these separate functions to be performed, as many as three separate current sensing relays were required (one to protect against nuisance tripping of oil pressure safety switch and two for sensing current in a three phase defrost circuit).
If the oil pressure safety switch was not included on compressors equipped with oil pumps and the oil pressure dropped below a predetermined point, a failure of the compressor would likely occur. This could damage other parts of the refrigeration system. On the other hand, if a compressor was also equipped with internal overload circuit breakers and these tripped, the oil pressure safety switch would also trip unless it was protected against this nuisance tripping. Generally the internal overload circuit breakers will reset automatically; however, the oil pressure safety switch will not. Unless an oil pressure safety switch is protected against nuisance tripping the perishable contents of refrigerated space will spoil upon prolonged loss of refrigeration.
In recent years there has been increased demand for a means of minimizing defrost times and reducing the amount of defrost heat added to the refrigerated spaces for both energy conservation reasons and, in the case of refrigerated foods, quality reasons. In answer to these customer demands, thermostats are increasingly being used on individual evaporator coils that disconnect the defrost heaters once the coil's temperature has been raised high enough to melt the accumulated frost. However, the compressor control panel has to have its own means of determining when to end the defrost cycle and start the refrigeration cycle.
The means of accomplishing this function of triggering the end of the defrost cycle that has increasingly been asked for by users is the current sensing defrost termination. Prior to the present invention the normal method for accomplishing this function was the use of one current sensing relay on single phase defrost circuits and two current sensing relays on three phase. These relays were used only to sense the current in the defrost circuit.
The use of individual defrost heater thermostats and current sensing defrost termination saves both electrical energy as well as prolongs the shelf life of refrigerated and frozen foods stored in systems utilizing these controls. One additional benefit is the labor saved by not requiring any additional refrigeration line control hookups or field installed control wires with the corresponding cost savings.
As described in the literature from the manufacturer of the current sensing relay, there is a problem with getting proper phasing between the powering circuit for the current sensing relay and the direction of flow of the circuit being sensed by passing through the sensing loop of the relay. If the phasing was incorrect, the current sensing relay would not activate even though there was sufficient current in the wire passing through the sensing loop of the relay. Prior to this invention, this phasing problem could cause difficulty for the installers of equipment having this type of current sensing relay.
Though it is uncommon, the internal overload circuit breakers for the compressor of a refrigeration unit may trip periodically. As soon as the compressor unit cools down the circuit breakers will close and normal operation of the refrigeration unit would resume if the oil pressure control switch has not opened; however, in most refrigeration units the oil pressure will have decreased to such an extent that unless protected against nuisance tripping the oil pressure control switch will have opened, thereby preventing the continuation of the refrigeration upon closing of the internal overload circuit breakers for the compressor. The present invention is designed to remove power from the oil pressure control switch if no current is flowing through the compressor and thus prevent nuisance tripping of the oil pressure control. This is accomplished through the current sensing relay which provides the additional function of triggering the time delay relay once the defrost cycle has been completed.
By using the present control circuit several different refrigeration units can be operated by the same controls. The same compressor may be supplying refrigerant to a number of refrigeration units with a separate defrost heater located in each unit. This would minimize the amount of controls that would be necessary and reduce costs in a facility such as a supermarket.